Dodge Durango Engine Temp Gauge Goes Full Hot At Times
A shop called me in to help with a pesky Dodge Durango. The customer complaint was that at times the temperature gauge goes hot. The shop had over the last 6 months replaced the sensor with first aftermarket then factory sensors with no resolution. Freeze frame data showed -41F at time of code setting. The assumption is that the gauge goes full hot in an open or short circuit to make sure the vehicle gets serviced. So the question then was, is it a connection, wiring or PCM problem.
In order to remove the sensor from the equation, I backprobed the ECT connector and used a decade box to simulate the thermistor contained in the sensor. At first it was quite confusing. With the sensor open the temp light inside went to full hot. The scan tool temp reading barely flinched. With very low resistance the result was the same. I switched to the DRBIII emulator tool in case the problem was with the aftermarket tool. Same readings. After looking over the diagrams of the system it was confirmed that the same sensor addressed the gauge and the PCM inputs. Then I noticed that the sensor voltage was changing drastically.
At 400K Ohms the voltage read 4.97 Volts. In this case the value seemed to be in range because it now read -43.6F. This was pretty much the limit where the temp worked correctly. When the resistance was increased, the voltage changed but the temp value appeared to be a substituted value. At 40K Ohms the temperature read 24.8F. At 400 Ohms the temp read 246F. Fewer Ohms than that and back to the substituted values.
Since the problem was intermittent, I decided to look for wire rubs or any indicator that would point to an open. I saw in the service information a splice pack for most sensor grounds. There was nothing disturbed with the loom. I then started to do a wiggle test with the scan tool screen in front of me. I grabbed the pcm wires just below the connector and gently moved the wires. And there it was. Voltage went from around 2.5V to 5.01V. Wiggle a little more and the value went back. In drag testing the terminal barrels, they all tested good. The problem is most likely a bad solder joint within the PCM. Luckily we were able to find a PCM at a local wrecking yard.